1. Technical Field
The present invention relates to a method and an apparatus for the correction of a frequency offset of signals in a multicarrier system. In particular, the present invention relates to a method and an apparatus for multicarrier signal frequency corrections which implement a prediction of frequency corrections to be carried out for received multicarrier signals or parts thereof. For that purpose, the present invention is further directed to a phase locked loop approach for a decision directed frequency synchronization in multicarrier systems.
2. Discussion of the Prior Art
Various methods for the transmission of digital signals, such as digital video broadcasting (DVB) and digital audio broadcasting (DAB) signals, are known. One method typically used for such transmissions is the orthogonal frequency division multiplexing (OFDM) method wherein a plurality of modulated signal carriers are used to broadcast the signals. Multicarrier modulation schemes as the OFDM are typically used in systems wherein the time dispersion thereof is much greater than the employed bit duration.
The modulated signal carriers are sampled before being transposed in the frequency domain by means of a fast fourier transformation (FFT) for signal separation. Due to frequency differences between transmitters and receivers in such systems, the demodulated signal carriers can exhibit frequency offsets.
Signal transmission standards, such as the high performance radio local area network type 2 (HIPERLAN/2), use coherent modulation schemes. For an assessment of frequency offsets of received signals and a frequency synchronization to be performed subsequently, so-called preambles are introduced into a data stream of the transmitted signals as training sequence. In case of the OFDM, two identical OFDM symbols (C64) are inserted between a cyclic prefix (C32) and the actual data stream. This so-called C-preamble shown in FIG. 1 is used, e.g. for a channel estimation in the demodulation process of the multicarrier signals.
In particular, the accuracy of signals in the HIPERLAN/2 standard leads to high frequency offsets. As a result, algorithms for a compensation of these frequency offsets have to be implemented. On the basis of the OFDM, usually a frequency correction is done based on an estimation of phase offsets using the C-preamble. The frequency offsets still remaining result in a low performance and require further compensation to correct the remaining frequency offsets.
A common approach for that purpose is to use a frequency tracker employing a phase locked loop (PLL).
Frequency correction means on the basis of first order phase locked loops, such as disclosed in EP 656 706 A2, leave a remaining phase offset for demodulated signals which causes further signal errors upon applying higher order modulation schemes. Here, an improvement can be achieved if measures for a forward phase correction are taken. On the other hand, such a forward phase correction results in a higher complexity for these frequency trackers.
Another approach utilizes second order phase locked loop frequency trackers due to their ability to eliminate remaining phase offsets. A disadvantage of second order phase locked loops is the increased acquisition time leading to error propagations due to the feedback loop. The acquisition time can be reduced by means of increasing the bandwidth of the phase locked loop. In single carrier systems, this results in a low noise suppression. In contrast thereto, in multicarrier systems, e.g. employing the OFDM, the noise suppression due to an increased bandwidth can be achieved by an averaging process in a demodulator for received multicarrier systems.
As disclosed for example in EP 817 418 A1, the demodulator, e.g. a fast fourier transformation means, averages over several subcarriers already leading to a sufficient noise suppression. Therefore, a reduction of the bandwidth is here not required for improving the noise suppression. Since the averaging process in the demodulator requires a block processing structure, a time delay is inherent. This leads to a greater acquisition time.
Common to frequency corrections for multicarrier signals is, beside the correction of the frequency, a derotation of the phase of the signals. This effects that there does not remain a phase offset. Since such feedback loops incorporate a double integration property, a frequency correcting control signal can be applied to received multicarrier signals even when no phase offset is present after a setting time.
Therefore, the object of the present invention is to provide a solution for a frequency correction in a mulitcarrier system which utilizes the benefits of a second order phase locked loop and, in addition, overcomes the problem of a great acquisition time in the prior art.